Access devices for measuring temperature of a patient

ABSTRACT

An access device such as a catheter, or introducer, or any combination of the above is provided. Within the access device is at least one lumen, channel or instrument that carries or itself is a thermally active mass, such as infusion fluids, control wires, etc. A temperature sensor such as a thermistor is secured to the access device in order to measure the temperature of a temperature medium, typically blood, in a patient. Various insulating lumens, insulating members and mounting and extrusion configurations are provided by the invention to insulate the temperature sensor thermally from the thermal mass, which might otherwise degrade the accuracy of the temperature measurement. The invention also provides an arrangement whereby the temperature sensor is connected to an external monitor for display of the patient&#39;s temperature.

RELATED APPLICATION

[0001] The present application is a continuation of U.S. Pat. No.6,383,144, filed Jan. 18, 2000, and scheduled to issue on May 7, 2002.

FIELD OF THE INVENTION

[0002] This invention relates to methods and devices for measuring thebody temperature of a patient in conjunction with the placement withinthe patient of an access device, for example, a catheter or introducer.

DESCRIPTION OF THE RELATED ART

[0003] The needs to properly treat a patient and to gain as muchinformation as possible about the physiological state of a patient areoften at odds with the desire to reduce discomfort to the patient asmuch as possible. For example, there is frequently a need both todeliver various medications to a patient, and also to monitor thepatient's body temperature. Accordingly, catheters are often insertedinto the vasculature of a patient to allow delivery of variousmedications, hydrating fluids, etc., and to measure blood pressure. Thepatient's body temperature, however, is monitored with a separatedevice, which is inserted separately.

[0004] Conventional devices for measuring temperature include thewell-known oral thermometer, rectal, axillary (armpit), and tympanic(ear) thermometers and probes, as well as Foley catheters (bladdertemperature), and nasopharyngeal probes (esophagus) probes. Each ofthese devices suffers from one or more shortcomings. The firstdisadvantage is obvious to anyone who has ever been the patient: It isuncomfortable enough to have a catheter inserted into one's vein orartery without also having to have a separate device inserted into one'srectum, bladder, ear or nose, or down one's throat. having to have aseparate device inserted into one's rectum, bladder, ear or nose, ordown one's throat.

[0005] The second disadvantage has to do with accuracy—taking apatient's temperature by placing a thermometer under her armpit or inher mouth may cause the least discomfort to the patient, but thetemperature value this provides is usually less accurate and much moredependent on placement than temperature measurements of blood in a majorvessel.

[0006] One way to overcome these disadvantages is to include some formof temperature sensor within the inserted catheter itself. This allowsfor measurement of the blood temperature, which is in most cases muchcloser to the patient's actual body core temperature. The problem thenarises that other elements of the catheter system may have thermalproperties that themselves affect the temperature that the sensorsenses. This problem arises in the context of thermodilution systems formeasuring cardiac flow. U.S. Pat. No. 4,817,624 (Newbower, Apr. 4,1989), U.S. Pat. No. 5,176,144 (Yoshikoshi, Jan. 5, 1993), and PublishedEuropean Patent Application 0 357 334 B1 (Inventors: Williams, et al.,Mar. 7, 1990) for example, describe such systems. As is well known, insuch a thermodilution system, the temperature of the cardiac blood flowis modulated according to a predetermined pattern that is created by theinjection of an indicator, which is usually either a series of bolusesof a relatively colder fluid, or heat. The downstream response to thetemperature modulation is sensed by a thermistor and is used tocalculate and estimate blood flow.

[0007] In systems such as Newbower's, temperature modulation isaccomplished by cooling the blood through precisely dosed boluses of athermally well-controlled fluid colder than the blood. In Williams,modulated cooling of the blood is accomplished using a heat exchangemechanism that does not require actual injection of any bolus into theblood stream. In systems such as Yoshikoshi's the blood is insteadheated locally using a heating element that is mounted near the far(distal) end of a cardiac catheter. As before, a thermistor senses thedownstream response profile, whose characteristics are used to calculatecardiac flow.

[0008] Such thermodilution systems have certain clinical limitations,since they must deal with several problems specific to this application.First is the problem of retrograde flow: If the thermistor is locatedproximal to the heater or bolus injection port, then the heated/cooledblood will flow back over the catheter tip. The temperature of thecatheter itself, which may contain various other lumens, injectates,control wires, etc. can then affect the temperature profile of thethermally modulated blood and degrade the flow calculations.

[0009] To overcome this effect, the injection is replaced by acontinuous infusion of indicator in order to obtain a new steady-statebaseline; however, this is an undesirable clinical limitation due to thevolume-loading the patient. Even when the thermistor is located distalrelative to the heater/bolus port, this problem may still arise.

[0010] These thermodilution system catheters normally have a distalinfusion lumen that passes beneath the thermistor or temperature sensorand exits at the tip of the catheter. Since the flow in such an infusionlumen can severely degrade the accuracy of the temperature sensormeasurements, the flow is limited to a maximum amount in order for theblood flow measurement to still be accurate. Of course, such alimitation on infusion lumen flow is also undesirable from the clinicalperspective.

[0011] An analogous problem of insulation arises in other cardiacdevices as well, such as the catheter-based cardiac ablation systemdescribed in U.S. Pat. No. 5,688,266 (Edwards, et al., Nov. 18, 1997).In Edwards' system, an ablation electrode is used to kill tissue locallyusing heat, and one or more temperature-sensing elements are used tosense the temperature of the tissue to be ablated and allow precisecontrol of the ablation temperature and time. Isolation, providedprimarily by physical separation, is thus required between the electrodeand the temperature sensors; otherwise, the sensors will tend to givereadings that are too high.

[0012] At least one factor limits the use of these known systems ingeneral use for measuring a patient's body temperature: These systemsare not arranged to measure the patient's actual, natural bodytemperature at all, but rather the temperature of blood or some bodytissue whose temperature the system itself has deliberately altered.

[0013] There are other devices, such as central venous catheters (CVC),peripheral catheters, and other catheter-like instruments such asintroducers. As their names imply, such catheters do not requireplacement into the heart and are consequently used more frequently indifferent areas of the hospital. Unlike cardiac catheters, which areoften more than 100 cm long and require an introducer for insertion,these devices are seldom longer than about 20-30 cm and can be insertedby the Seldinger technique. A CVC, for example, is often placed in apatient's jugular vein and is used for various infusions, for monitoringblood pressure, etc., through a number of lumens within the device.

[0014] An instrument such as a CVC often includes several differentlumens which may carry a range of fluids (such as medications and otherinfusions), as well as instruments such as pressure transducers. Each ofthese fluids and instruments may be at different temperatures, or mayhave varying thermal properties, or both. Any measurement of temperatureusing such a catheter would thus risk serious thermal contamination fromother portions of the catheter.

[0015] There are at present no known devices such as a CVC, peripheralcatheter, or introducer that incorporate an arrangement for measuringblood temperature accurately. Therefore, it would be advantageous to beable to accurately measure temperature in conjunction with such accessdevices as catheters and introducers while eliminating the need toinsert a secondary device into the patient in order to measuretemperature, as is the current practice. Such devices would also providea more accurate and less time-consuming body temperature measurementthan non- or less invasive devices. This invention provides such anarrangement.

[0016] It would also be advantageous to be able to connect a CVC orsimilar catheter to a standard patient monitor. Not only would thisbring the obvious benefit that the patient's temperature could be viewedat a glance along with other monitored parameters, but it would alsomake the temperature values available for other processing as needed.Many patient monitors, however, use a signal standard that is compatiblewith large thermistors or temperature sensors and not compatible withthe output of miniature temperature sensors used on pulmonary arterycatheters. The use of miniature thermistors is desirable because itallows for catheter sizes to be relatively small. One could of coursereprogram the monitors, but such a solution to the problem would becostly and complicated, and may not be possible or practical in existingmonitors. This invention provides an arrangement that allows acatheter-based temperature sensor to be connected to existing monitors.

[0017] An additional issue is that many patients, as their conditionimproves, do not require continuous monitoring of temperature, andtherefore, do not require a dedicated connection between the catheter(s)and the monitor. At present, the dedicated connections limit how manypatients the system can monitor, and increases the number of cables andconnectors needed. It would be advantageous to free the system to allowmonitoring more that one patient. This would, for example, enable nurseor physician to have a quick look at the patient's temperature, possiblyenter it into the patient's chart, and then move on to other tasks orpatients. It would therefore be beneficial to have an arrangement thatprovides this flexibility and simplicity. This invention does this aswell.

SUMMARY OF THE INVENTION

[0018] In general, the invention provides an access device, such as acatheter, an introducer, or combination of catheters, introducers,probes and the like, that allows more accurate sensing of bodytemperature, for example, of a temperature medium such as blood, byinsulating a temperature sensor from thermal contamination caused by athermal mass, such as an infusion fluid or an instrument, introduced inportions of the access device. In the preferred embodiment of theinvention the access device is a central venous device that includes atemperature sensor such as a thermistor, a thermocouple, etc.

[0019] The access device is insertable into the patient at a location ofthe temperature medium, and the access device includes at least onethermal mass other than the temperature medium. The access devicesupports the temperature sensor and includes at least one insulatingstructure insulating the temperature sensor from the thermal mass.

[0020] In certain embodiments of the invention, each thermal mass islocated within a thermal lumen within the access device. The temperaturesensor may be mounted externally to an outer surface of the accessdevice, or within a sensor lumen of the access device. The insulatingstructure preferably extends between the temperature sensor and eachthermal lumen.

[0021] The temperature sensor may also be mounted in or on a carrier.The insulating structure is then preferably formed as a barrier withinthe carrier and the carrier is held in one of the lumens of the accessdevice with the barrier extending between the temperature sensor and thethermal lumen. The carrier may be removably insertable in the lumen ofthe access device.

[0022] In other embodiments of the invention, a pair of ports is formedin an outer wall of the access device and a flow channel is formedwithin the access device and extends between the pair of ports. Thetemperature medium, such as blood, then occupies the flow channel. Theflow channel is located between the temperature sensor and the thermallumen, or between the insulating structure and the thermal lumen, andthereby not only increases thermal contact between the temperaturesensor and the temperature medium, but it also thermally isolates thetemperature sensor further from the thermal lumen. The flow channel maythus itself form the insulating structure.

[0023] In another embodiment of the invention, the access device has anopening in an outer wall and the temperature sensor, when in a deployedposition, extends into the opening. This increases thermal contactbetween the temperature sensor and the temperature medium and furtherinsulates the temperature sensor from the thermal mass. If thetemperature sensor is mounted on a carrier, then ends of the carrier maybe secured within the access device. The carrier is then positionedbetween the temperature sensor and each thermal lumen, thereby formingthe insulating structure.

[0024] The temperature sensor may alternatively be mounted within thecarrier, which then protrudes as a loop out through the opening in theouter wall of the access device. The ends of the carrier are thenpreferably secured within the access device. In this embodiment, theinsulating structure comprises a flow channel for the temperaturemedium, which is formed between the carrier and the access device at theposition of the opening, and thus between the temperature sensor and thethermal mass. One advantage of this embodiment is that the temperaturesensor is exposed substantially over its entire outer circumference tothe temperature medium, via only the carrier.

[0025] Alternatively, the temperature sensor may be a right-anglethermistor mounted to extend out of the opening mainly perpendicular toa central axis of the access device.

[0026] In another embodiment of the invention, the temperature sensor isadhesively attached to the access device. The adhesive may bedissolvable at body temperature, so that the temperature sensorseparates from contact with the access device when in position withinthe patient.

[0027] The access device may include a plurality of lumens, whereby thetemperature sensor is mounted within a recess in an insulating member.The insulating member, together with the temperature sensor, are thenmounted within one of the lumens of the access device so that theinsulating member extends between the temperature sensor and the thermallumen.

[0028] In another embodiment of the invention, the insulating structureincludes an insulating material that is co-extruded with the accessdevice and surrounds either at least a portion of each thermal lumen, orthe temperature sensor itself.

[0029] In yet another embodiment of the invention, the access device hasa lumen and a sensor port and the temperature sensor is mounted on adistal tip of a separate device, for example, a probe. The probe isinsertable into the lumen of the access device so that the temperaturesensor extends through the sensor port.

[0030] The insulating structure may also comprises a distal tip of theaccess device itself. The tip is then preferably formed from aninsulating material as a separate member, and the temperature sensor ismounted within the distal tip. Alternatively, the distal tip of theaccess device may be provided with a lengthwise extending slit. Thetemperature sensor is then mounted on a first side of the distal tip andat least one thermal lumen carrying the thermal mass extends through asecond side of the distal tip. The distal tip, in a deployed position,then separates along the slit, with the first and second sides of thetip being located on either side of the slit.

[0031] In another embodiment of the invention, the insulating structureis a lumen or a chamber in the access device that is expandable toincrease the distance between the temperature sensor and the thermalmass.

[0032] The access device according to the invention is preferablyincluded as a sensing member in a more general system for monitoring thebody temperature of a patient. In this system, the access device isinsertable into the patient and is connected to a temperature monitorthat converts a sensor output signal of the access device into a patienttemperature signal and for displaying the patient temperature signal. Aconnector is then provided to connect the temperature sensor with thetemperature monitor.

[0033] The system according to the invention preferably further includesan adapter in the temperature monitor. The adapter converts the sensoroutput signal into a predetermined display format. The temperaturemonitor may also be provided with a display and a power supply, in whichcase the entire monitoring system may be implemented as a hand-held,self-contained unit that is portable between different patients.

[0034] The invention also encompasses a method for measuring the bodytemperature of the patient. The main steps of the method according tothe invention involve supporting the temperature sensor on the accessdevice; inserting the access device into a blood vessel; introducing atleast one thermal mass into the access device; and insulating thetemperature sensor from the thermal mass. In the preferred methodaccording to the invention, the thermal mass is introduced via a thermallumen located within the access device. One then mounts the temperaturesensor in a sensor lumen within the access device and forms at least onethermally insulating structure between the temperature sensor and thethermal lumen. In some embodiments, to provide the thermally insulatingstructure, one may introduce a thermally insulating material into alumen within the access device.

[0035] The invention also comprises a method for manufacturing theaccess device. In the preferred embodiment, this method comprisesextruding the access device, forming a thermal lumen through which athermal mass is introduced, forming a sensor lumen through which atemperature sensor is introduced, and forming an insulating structureseparating the sensor lumen from the thermal mass. In manufacturing theaccess device, the temperature sensor may be mounted in the sensor lumenat a distal end of the access device. A signal wire is then drawn fromthe temperature sensor to an external patient monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 illustrates one example of an access device according tothe invention, such as a CVC catheter, that is inserted into a patient'svein for measuring temperature.

[0037]FIG. 2 illustrates another example of an embodiment of theinvention in which a temperature sensor is located within a lumen of acatheter but is thermally insulated from other lumens by an insulatinggap.

[0038]FIG. 3a illustrates a temperature sensor that is provided within adedicated tubular member that also includes a built-in insulating lumen.

[0039]FIG. 3b shows the lumen of FIG. 3a in place in the catheter.

[0040]FIGS. 4 and 5 show embodiments of the invention in which blood isallowed to flow past the temperature sensor in place in the catheter,with and without an insulating gap being provided between thetemperature sensor and catheter lumens.

[0041]FIGS. 6a and 6 b are side and end views, respectively, of anotherexamplary embodiment of the invention in which the temperature sensor ismounted on an insulating member, whereby both are inserted into the samecatheter lumen.

[0042]FIGS. 7a and 7 b are side and end views, respectively, of anembodiment of the invention in which the temperature sensor is mountedon the outer surface of the catheter.

[0043]FIGS. 8a and 8 b are side and end views, respectively, of anotherembodiment of the invention in which the temperature sensor is mountedto extend out from the outer surface of the catheter, with a blood flowchannel located between the temperature sensor and the outer surface.

[0044]FIG. 9 illustrates an embodiment of the invention in which thetemperature sensor is a right-angle thermistor extending through anopening in the outer surface of the catheter to provide surface contactbetween the temperature sensor and the blood.

[0045]FIGS. 10a and 10 b illustrate an embodiment of the invention inwhich the temperature sensor is mounted on an a separate insulatingmember that can be inserted along with the sensor into a catheter lumen.

[0046]FIG. 11 illustrates an embodiment of the invention in which thetemperature sensor is mounted on the tip of a probe that can be insertedinto an access device such as a catheter.

[0047]FIGS. 12a and 12 b illustrate embodiments of the invention inwhich an insulating material is co-extruded with the catheter itself.

[0048]FIGS. 13a and 13 b illustrate another embodiment of the invention,in which the temperature sensor is mounted within a catheter tip that isinitially formed as a member separate from the catheter body itself.

[0049]FIGS. 14a and 14 b illustrate still another embodiment of theinvention, in which the distal tip of the catheter splits after it isplaced within the patient, with the temperature sensor and catheterlumen(s) containing thermal mass then deployed on either sides of thesplit.

DETAILED DESCRIPTION

[0050] In broadest terms, this invention provides an arrangement or adevice in which a temperature sensor is used with an access device,preferably a vascular access device, for insertion into the body of apatient. This invention also provides various insulating structures thatreduce thermal contamination of the temperature sensor from otherportions of the interior of the access device. The temperature sensor isdesigned to sense some temperature medium within the patient's body, forexample, blood.

[0051] One example of the preferred access device of this invention is acentral venous catheter (CVC), but it could be some other instrumentthat also carries or includes fluids or other devices—cumulatively“thermal masses”—that could affect the temperature at the temperaturesensor. Examples of other access devices include peripheral catheters,introducers, obturators, and probes. In fact, the term “access device”also contemplates any combination of these devices, such as acombination of one or more introducers, catheters and probes. Forexample, a catheter is often inserted within an introducer, and eitheror both could be arranged according to suitable embodiments of theinvention to improve the accuracy of temperature measurements.

[0052] In the context of this invention, a thermal mass is any substanceor structure carried within the access device that has or could have atemperature and heat capacity such that heat flow into or out of themass could significantly affect the sensed temperature. Here,“significantly” means so much that the temperature measurement would notbe acceptably accurate for clinical use.

[0053] As used in this invention an “insulating structure” is anystructure that insulates the temperature sensor from a thermal mass. Asis described and illustrated below, insulating structures used in theinvention, include, but are not limited to, a device lumen or anyportion of a device lumen, a channel, a gap, a chamber or just an areaprovided immediately surrounding the temperature sensor. An insulatingstructure may also include an insulating material, for example, aceramic, or a separate device such as a probe that is inserted into orthrough the access device.

[0054] The examples of suitable access devices described below arepreferably made of biocompatible polymer materials, since in most casesthey will be inserted at least partially into a patient. Polyurethane isthe most common material, since it meets all normal requirements forthermal and mechanical stability when in a patient; PVC and Teflon arealso acceptable, as well as other conventional materials. The accessdevices for use with this invention may, moreover, be made of ananti-microbial material or may be covered with material or coatinghaving anti-microbial or thromboresistant properties.

[0055] The temperature sensor used in this invention may be anyconventional device. The most easily implemented sensor is a thermistor,which is small, widely available and relatively easy to calibrate. Othertemperature sensors may, however, also be used. Alternatives includeconventional thermocouples and fiber optic temperature sensors. The onlyrequirement is that the sensor should predictably change a measurablephysical property, such as its electrical resistance or opticalspectrum, in response to changes in temperature, and this change shouldbe detectible externally via an electrical or optical conductor in sucha way that temperature can be converted to an electrical signal. Thesedevices, and the way in which their signals are conditioned for furtherprocessing are well known.

[0056] In the following discussion of the various exemplifyingembodiments of the invention, it is assumed merely by way of examplethat the access device is a CVC, that the temperature sensor is athermistor, that the catheter is inserted into a body vessel, such as avein, and that the temperature medium whose temperature is to bedetermined is blood. The invention will work just as well with otheraccess devices and sensors, insertion points, and temperature media, aswill be obvious to those skilled in the art.

[0057]FIG. 1 illustrates the general structure of the invention. Acatheter 100 is inserted into a patient's vein 110 in the conventionalmanner. Arrows within the vein 110 indicate flowing blood. A thermistor120 is positioned at the distal end of the catheter, which includeslumens, channels or tubes through which fluids can be infused into thepatient, or which hold other instruments. Two conventional infusionconnectors 130, 132, are shown inserted into respective lumens in thecatheter. The number of lumens and connectors will of course depend onthe particular catheter used and the application. The invention willwork with any number of lumens or internal channels in the catheter.

[0058] A conductor (shown as the dashed line 125), which forms a signalwire, connects the thermistor electrically (or optically, depending onthe type of temperature sensor used) with external conditioning,processing and display circuitry 150. In FIG. 1, this exemplarycircuitry is shown as including a signal adapter 160 and a patientmonitor 170, with a conventional electrical coupler 180 and a guide tube185 connecting the thermistor signal wire 125 to the external circuitry150. A conventional power supply 172 is also included, as is atemperature display 174, which may be either a separate display deviceor simply a portion of an existing monitor display. These features, someof which are optional or can vary depending on the embodiment, aredescribed below in greater detail. Any conventional devices and circuitsmay be used to communicate the thermistor's 120 output signal toexternal monitors or displays.

[0059]FIG. 1 also shows a section line A-A. The description of variousembodiments of the catheter according to the invention is illustrated bycross-sectional drawings. Line A-A is the reference line for thesecross-sectional views.

[0060]FIG. 2 illustrates one exemplifying embodiment of the invention.In this embodiment, the thermistor 120 is located within a dedicatedopening or lumen 210 within the catheter 100. In this figure, thethermistor lumen 210 is shown as being mainly circular. This is notnecessary; any appropriate and desired lumen shape may be used. Acircular or at least rounded lumen cross section will in most cases bepreferable, however, since standard thermistors frequently are providedas glass-encapsulated beads with a mainly round cross section. Threeother lumens 220, 222, 224 are also illustrated (however, any number oflumens may be included).

[0061] Assume now that one or more of the lumens 220, 222, 224 carriessome fluid (or contains some instrument) with a thermal mass andtemperature that could affect the temperature measured by the thermistor120. For example, an infusion fluid might be administered through thelumen 220. If the temperature of the fluid is above or below that of thepatient's blood, then it could influence the temperature measurementbecause of the thermal conductivity of the catheter material between thethermistor and the fluid. An additional insulating structure, such as alumen or gap 250 is therefore preferably extruded in the catheter so asto extend, for example, laterally between the thermistor and all theother lumens 220, 222, 224.

[0062] The insulating lumen (gap) 250 is preferably as wide and thick aspossible to maximize the degree of thermal insulation of the thermistor,given the minimum permissible material thickness required to maintainstability of the catheter and lumen walls, as well as the maximum outerdiameter of the device. The minimum distance between the thermistorlumen 210 and the outer surface of the catheter 100 is, however,preferably as small as possible to ensure the best thermal contactbetween the thermistor and the surrounding blood.

[0063] The insulating structure, such as the lumen or gap 250 of FIG. 2is preferably filled with air, or with some other conventional gas,ceramic pellets, a conventional high-impedance gel, etc., toadditionally increase its thermal impedance. The insulating material mayalso be a strip or layer or similar separate piece of an insulatingmaterial that is inserted into the lumen 250. This insulating materialmay optionally be bonded to the catheter in any known way. The mostdistal end of the insulating lumen is preferably sealed to preventinflow of blood and outflow of the thermally insulating gas or otherinsulating material.

[0064] In FIG. 2, only one insulating lumen is shown. This is by way ofexample only. More than one gap may be created, space permitting, toextend between the thermistor and the other lumens to further increasethe thermal isolation of the thermistor. Also, the insulating lumen maybe of any length—it may extend through the full length of the accessdevice or any appropriate portion of its length. For example, a portionof the lumen 250 may be used as an infusion or device lumen forintroduction of medications or guidewires. A plug may be placedsomewhere along the length of such lumen to block off the remainder ofthe infusion/device lumen so that the remaining portion will act as aninsulating structure. The location of the plug must be selected suchthat the blocked off portion of the infusion/device lumen will beadjacent to the location of the temperature sensor. It will be necessaryto provide a side port prior to the location of the plug to allow theinfusion/device to exit the access device.

[0065] The lumen(s) 250 also does not need to be shaped as a generallylaterally extending slit, as shown in FIG. 2, although this typicallymaximizes the isolation of the thermistor from the other lumens.Instead, lumen 250 may be shaped as half-moon or be concentric with thethermistor lumen, or otherwise extruded so as to surround the thermistorlumen 240. Also, the gap could be created by several mainly cylindricalor otherwise curved lumens spread out between the thermistor and theother lumens 220, 222, 224.

[0066] In yet another variation of the insulating lumen 250 it—that is,the catheter material around and defining it—is made elastic enough thatthe lumen 250 is inflatable after the catheter is inserted into thepatient. For example, the lumen 250 could be formed to have flexiblewebs. Once the catheter is inserted, any suitable pressurizing material,such as air, an inert gas, foam, or some other known thermal resistancematerial could be pumped into the lumen 250, causing its cross-sectionalarea to expand and increase the gap or distance between the thermistorand thermal masses. The embodiment facilitates easy insertion of thedevice by keeping its outer diameter small, since the insulating lumenor structure is expanded only after the device is in place.

[0067] The lumens 220, 222, 224 may be used for any conventionalpurpose. Any or all of them may, for example, carry fluids, or act aschannels for guiding other instruments such probes, pressuretransducers, etc. Of course, they need not all have the samefunction—one lumen might be carrying an infusion fluid while another isa channel for an instrument.

[0068]FIGS. 3a and 3 b illustrate an embodiment of the invention inwhich the thermistor 120 and a thermally insulating lumen/gap 350 areprovided in a separate mainly tubular member 300 which may be insertedinto an existing lumen 310 or channel within the catheter 100. Thetubular member 300 is preferably made of the same—or at least same typeof material as the catheter itself, that is, a thermally stable,biocompatible polymer such as polyurethane. This material requirement isnot as strict as for the catheter itself, however, since the tubularmember is mounted within the catheter. The gap 350, which may be filledwith further insulating materials as described above for the lumen 250,is then oriented within the lumen 310 so as to extend between thethermistor and other lumens 320, 322, 324, 326 within the catheter. Inorder to provide proper orientation of the tubular member within thelumen 310, a key (not shown) such as a rod shaped to conform to the gap350 could be provided, if needed. The user can then first insert themember 300, with the thermistor, into the lumen 310 and then insert thekey into the proximal end of the gap 350 and turn the member 300 intoproper alignment.

[0069]FIGS. 4 and 5 illustrate embodiments of the invention in whichblood itself is channeled between the thermistor 120 and one or moreother lumens 424, which may be carrying sources of thermal “noise” suchas infusion fluids. In these embodiments, ports 410, 412 are formed inmainly diametrically opposing portions of the outer wall of the catheter100 and a channel is formed (as part of the normal extrusion between thetwo ports). The ports 410, 412 may be arranged anywhere along thecircumference of the catheter wall—not just diametrically opposing—aslong as blood can flow between the temperature sensor and the thermalmasses. In FIG. 4, the channel has three chambers—two outer chambers440, 444 and an intermediate chamber 442—through which blood can flow(indicated by arrows passing though the channel). Note that the ports410, 412 need be formed only in the region of the thermistor 120, andcan thus be simple holes or slits cut in the catheter wall. The channelmay be formed as a small chamber or it may extend over any length of thecatheter as a result needed to simplify the extrusion. Note that a CVCor peripheral catheter, unlike a cardiac catheter, is typically no morethan about 30 cm long, so it will in general not be a problem to let thechannel extend as far as the other lumen(s) 424.

[0070] In the embodiment of the invention shown in FIG. 4, the blood isdirected to a region—the intermediate chamber 442—immediately adjacentto (that is, extending just under, viewed as in FIG. 4) the thermistor120; the maximum distance separating the thermistor from blood whosetemperature is to be measured both above and below can be made as littleas the minimum structurally allowable thickness of the cathetermaterial. The blood thus not only helps isolate the thermistor from thelumen(s) 424, but it also better contacts the thermistor thermally,since it does so from two sides instead of just one. A central ridge ortab 470 may be extruded to extend between the two outer chambers 440,444 and from the lumen 424 toward the thermistor, in order not only todirect the inflowing blood past the thermistor, but also to reduce theamount of blood within the catheter while still allowing for aninsulating layer of blood to flow between the thermistor and thelumen(s) 424. The ridge is, however, not necessary to this embodiment ofthe invention.

[0071] In the embodiment of the invention illustrated in FIG. 5, thechambers 440, 444 and 442 and the ridge 470 (FIG. 4) have beeneliminated. Instead, the intermediate chamber 442 is sealed off from theblood flow and thus forms an insulating gap or lumen 550, similar to thelumen/gap 250 in FIG. 2. In this embodiment, the blood flowing throughthe single channel 540 serves mainly to isolate the thermistor thermallyfrom the lumen(s) 424. The lumen/gap 550 provides an additionalinsulating barrier, although it is not required, especially if the flowof blood through the channel is fast enough to preclude significant heattransfer to or from the thermal mass from which the channel separatesthe thermistor. Note that another advantage of the embodiment shown inFIG. 5 is that the blood in the channel 540 also tends to bring thetemperature within the gap 550 to blood temperature and thus furtherinsulates the thermal mass.

[0072] In the embodiments of the invention shown in both FIGS. 4 and 5,the channel 540 may be a limited chamber located near the thermistoritself, or it may be a lumen passing through any portion of the lengthof the access device. In either case, the channel 540 itself (withpassing blood) serves as an insulating structure.

[0073]FIGS. 6a and 6 b are a partially cut-away, side view and an endview, respectively, of another embodiment of the invention in which thethermistor 120 is mounted on a carrier 600, which is preferably made ofa biocompatible material and also provides improved thermal insulation.It may be made, for example, of plastic, metal or ceramic. Thethermistor may be mounted securely onto the carrier using anyconventional material such as a standard adhesive such as pottingcompound or a non-toxic, moisture-proof, thermally stable glue.

[0074] In this embodiment a port is formed as a cut-away opening 605 inthe outer wall of the catheter 100. The thermistor is then positioned soas to lie within the opening in the catheter and thus be exposeddirectly to the blood over most of its surface are, without any portionof the catheter in between. The thermistor's signal wire 125 is alsoshown in FIG. 6a.

[0075] The thermistor 120 and its carrier 600 may be inserted into anexisting or dedicated lumen 610 in the catheter so that the carrierextends between the thermistor and other lumens 620, 622 or thermalnoise sources in the catheter. Note that the opening 605 preferablyextends into the lumen 610 to ensure maximum direct contact between thethermistor and the surrounding blood.

[0076] The thermistor and carrier 600 may be inserted into the catheterwith the thermistor in position in the opening 605 before the catheteris placed within the patient. Alternatively, before insertion, andassuming the carrier is made of a sufficiently flexible material, thethermistor and the far, distal end of the carrier 600 could be allowedto stick out of the opening 605, preferably bent back along the catheterwall and pointing away from the direction of insertion. Once thermistorcatheter is placed in the patient, the physician could then pull on theproximal end of the carrier until the thermistor is pulled into place inthe opening 605. The distal end of the carrier can then be made short,extending only a short distance from the thermistor, so that only itsproximal end would be within the catheter. The carrier, which may betubular, then forms an insulating gap beneath the thermistor, similar tothe gaps 250, 350 and 550 in previous embodiments described above.

[0077]FIGS. 7a and 7 b are a partially cut-away, side view and an endview, respectively, of an embodiment of the invention in which thethermistor 120 is mounted on the outer wall of the catheter 100 itself.In order to avoid having the thermistor's signal wire or fiber 125running along the outer surface of the catheter to the exterior, it ispre-threaded into the catheter 100 through a small hole 705 made in thecatheter wall, preferably just behind (proximal relative to) thethermistor 120. The thermistor may be mounted securely onto the catheterusing any conventional method or material such as a standard pottingcompound 710, or a non-toxic, moisture-proof, thermally stable glue, ora liquefied solution of the catheter material that would solvent bond tothe catheter tubing. The potting compound should be spread to cover thehole 705 and at least most of the thermistor, but not so thickly overthe thermistor as to interfere with its ability to quickly andaccurately respond to temperature changes. In order to reduce themaximum diameter of the catheter and thereby make insertion easier, anindentation could be made in the outer wall of the catheter. Thethermistor can then be mounted on the catheter by potting it securely inthe indentation (not shown).

[0078] In the embodiment of the invention shown in FIGS. 7a and 7 b, itwould also be possible to mount the temperature sensor using a non-toxicpotting material (or other adhesive) that dissolves when exposed to theblood. Once the catheter is in place, the potting material wouldtherefore dissolve. This would expose the temperature sensor directly tothe blood and thus allow for even more accurate temperaturemeasurements. Moreover, the temperature sensor will then tend toseparate and move away from the outer wall of the catheter, therebyfurther insulating it from any thermal masses within the catheter.

[0079] This “deployment” action may also be arranged by providing thesignal wire with an elbow joint made of a memory metal that is straight(extending in the direction of the catheter) during inserting but thatis bent in the relaxed state—when the potting compound dissolves, thejoint would relax and bend, thus moving the temperature sensor out fromthe catheter wall. If it is not practical to form this memory elbowjoint in the sensor's signal wire itself, then a piece of memory metalcould be attached to the wire where the elbow joint is needed. Thesensor could then also be potted within an indentation such as in FIG.6a, so that the catheter could have an outer surface free ofprotrusions.

[0080] As FIGS. 7a and 7 b show, several lumens 700-705 or tubularmembers are preferably included within the catheter in order to provideinsulating gaps between the externally mounted thermistor 120 and thelumen(s) that carry infusions. A single lumen/gap such as the lumen 250shown and described in reference to FIG. 2, or a blood channel similarto the channels shown in FIGS. 4 and 5 may be included instead of or inaddition to the lumens 700-705 to further insulate the thermistorthermally from the lumen 724.

[0081]FIGS. 8a and 8 b are a partially cut-away, side view and an endview, respectively, of an embodiment of the invention in which thethermistor 120 is mounted within a short tubular member 800 thatprotrudes out through an opening 805 made in the outer wall of thecatheter 100. The two ends of the tubular member 800 are secured withinthe catheter using any known technique. A channel 810 is thereby formedbetween the “loop” of the tubular member 800 and the catheter. Bloodwill therefore be able to flow substantially completely around thethermistor 120 and will also isolate the thermistor thermally from anyinterior lumen(s) 824 within the catheter. During insertion of thecatheter, the member 800 will preferably lie flat, that is, mostlystraight, within the catheter.

[0082] Once the catheter is in place, the physician could then insertthe thermistor, for example by pushing it in with a wire, and could thenpush the thermistor and loop of the member 800 out through the opening805 to deploy the temperature sensor, that is, the thermistor. One wayto do this would be to insert a separate instrument that has a bend onit into, for example, a lumen in which the member 800 lies (or simplythe interior of the catheter). Twisting the instrument with the bendunder the thermistor would then push it out through the opening 805.Alternatively, if the far distal end of the tubular member 800 is fixedin the catheter, and if the member 800 is not too flexible, then itwould push out through the opening by the physician pushing the proximalend inward.

[0083]FIG. 9 illustrates an embodiment of the invention in which thethermistor 120 is a right-angle device, that is, there is asubstantially right-angle bend in the rod or wire that connects it toits signal wire 125. Of course, angles of bend other than 90° may alsobe used—the proper angle of bend will depend on the particularimplementation and may be determined using known methods. Thisright-angle thermistor 120 is then potted securely in an opening 905,similar to the openings 605 and 805, formed in the catheter wall, sothat the thermistor extends outward approximately perpendicular to thedirection of longitudinal extension (central axis) of the catheter. Asbefore, the minimum amount of potting compound should be used to securethe thermistor, since this will also minimize the impact caused by thecompound itself on the thermistor's ability to sense blood temperature.As before, one or more insulating lumens 900 may also be included in thecatheter to isolate the thermistor from fluid-carrying lumen(s) 924.

[0084]FIGS. 10a and 10 b are a rear and an elevated side view,respectively, of an embodiment of the invention in which the thermistor120 is mounted so as to lie within a recess in a separate insulatingmember 1000, which is shaped generally as a partially hollowed outcylinder with a closed, rounded, smooth leading surface and a slot 1010into which the thermistor can be laid for mounting. The insulatingmember should be made of a smooth, thermally insulating material such asceramic, metal, foam or Teflon. Polymers such as polyurethane may alsobe used, which would make it possible to injection-mold the member 1000.The insulator/thermistor sub-assembly is then inserted, for example, bypushing it in with a rod, into a suitable catheter lumen, such as thelumens 210, 310, 610 shown above for other embodiments of the invention.The slot should thereby be oriented, for example, using a key or similartool, away from other catheter lumen(s) that carry thermal masses suchas fluids and instruments.

[0085] In FIG. 11, an embodiment of the invention is shown in which thetemperature sensor 120 is mounted on the tip 1110 of a separate device,for example, a guidewire or a probe 1100, which can be inserted into theaccess device 100. To deploy the sensor 120, once the access device isin place, the tip of the probe is inserted into a lumen of the device100 and is then pushed in until the probe tip 1110 protrudes from a port1140 that is either cut in the side wall of the catheter (as in some ofthe other embodiments described above), or is simply the innermostopening of the lumen in which the probe is inserted 1142. (Alternativeexit of the tip of the probe is shown as a dashed line.) The probe thusitself acts as a structure that separates (and thus insulates) thetemperature sensor from thermal masses. The tip of the probe ispreferably curved to a mainly “J”-shape so that it will more easilyextend through the port 1140 and away from the thermal influence of theparts of the access device; however, a straight tip is also acceptable.One advantage of this embodiment of the invention is that it could beinserted only if needed, in which case it can be sealed against bloodleakage by a conventional hemostasis valve.

[0086]FIGS. 12a and 12 b illustrate embodiments of the invention inwhich an insulating material is co-extruded with the catheter itself. InFIG. 12a, the insulating material 1200 is extruded along with thecatheter 100 so as to surround an infusion (or instrument-carrying)lumen 1210 or, alternatively, at least a portion of it near the locationof the temperature sensor. The insulating material, which may be of anyknown extrudable type then acts as a thermal barrier between thecontents of the lumen 1210 and the temperature sensor 120. In FIG. 12b,the insulating material is co-extruded with the catheter so as to form abarrier layer 1220 that surrounds and thereby insulates the temperaturesensor 120 itself.

[0087]FIGS. 13a and 13 b illustrate yet another embodiment of theinvention, in which the temperature sensor 120 is mounted within acatheter tip 1300 that is initially formed as a member separate from thecatheter body 100 itself, but is attached or bonded to the distal end ofthe catheter using, for example, a conventional adhesive. A lumen orthrough-hole 1310 is then formed in the tip 1300 to act as an extensionof any appropriate and desired lumen within the main catheter body 100to allow uninterrupted flow. The tip 1300 in this embodiment may then bemade entirely of a highly insulative material. This completely avoidsthe need to extrude the insulating member over much or even the entirelength of the catheter. It also makes possible the use of differentmaterials in the insulating member and the main catheter body with noneed for co-extrusion and without using more expensive material for theentire device.

[0088]FIGS. 14a and 14 b illustrate still another embodiment of theinvention, in which the distal tip of the catheter 100 has a slit 1400.The temperature sensor 120 is mounted on or in the distal tip on oneside of the slit, whereas the lumen(s) 1410 carrying the thermal massextend through the tip on the other side of the slit. In short, in thisembodiment, the distal tip of the catheter splits after the device isplaced within a patient. Before insertion into the patient, the cathetertip 1300 is held together either mechanically, for example, with aninternal catch that can be released using a wire that extends out of theproximal end of the catheter, or using an adhesive that dissolves whenexposed to blood, or any other appropriate method. While in place, theslit 1400 opens to form an insulating gap (as shown in FIG. 14b) betweenthe thermistor 120 and the thermal masses in the lumen(s) 1410.

[0089] Several different embodiments of the invention are describedabove. Common to all of the embodiments, however, is that they implementthe method according to the invention by which the body temperature of apatient is sensed by a temperature sensor supported by an access device.As used here, the term “supported” means that the temperature sensor maybe mounted on or within the access device; it may be permanently affixedto or within the access device; or it may be removably connected to orinserted into the access device. The term also includes any arrangement,as described for example in reference to FIG. 11, in which a temperaturesensor is located on a separate device, which is inserted into andextended through the access device.

[0090] The access device is inserted into a patient, for example, into avein, and at least one thermal mass is introduced into the accessdevice. The temperature sensor is insulated thermally from the thermalmass. A signal wire is led from the temperature sensor to an externalpatient temperature monitor.

[0091] The invention also encompasses the method of manufacturing theaccess device. In most of the embodiments described above, thismanufacturing method involves extruding the access device with aplurality of lumens—one lumen through which a temperature sensor isintroduced and a signal wire is led (a sensor lumen), and at least oneother lumen for carrying or guiding the thermal mass. The manufacturingmethod also includes the step of forming an insulating structure thatthermally separates the temperature sensor from the thermal mass. Thetemperature sensor may be permanently or removably mounted at a distalend of the sensor lumen. The temperature sensor may be also mounted in aseparate carrier which is placed in the sensor lumen. The manufacturingmethod may include some other or additional steps according to theembodiments described above, as will be understood by those skilled inthe art.

[0092] Refer once again to FIG. 1. The output signal from a conventionaltemperature sensor such as the thermistor 120 has well-knowncharacteristics. In general, the output signal is a voltage or currentsignal whose amplitude is functionally related to the temperature of thesensor. Moreover, the functional relationship between sensor temperatureand the amplitude of the output signal may be linear, but seldom is. Infact, most temperature sensors are individually calibrated by themanufacturer, or require calibration by the user before actual use.However obtained, there is, though, a functional relationship.

[0093] Furthermore, in some cases, the temperature output signal may becompatible with input signals of existing patient monitors, but this isnot always the case. As a simple example, amplification (scaling) andimpedance matching (or impedance isolation) are often required toconvert the output signal into a signal form and type that can beprocessed and displayed for the user.

[0094] According to the invention, the functional relationships a)between sensor temperature and the sensor output signal, on the onehand; and b) between output signal characteristics (such as impedance,amplitude range, and whether in the form of a voltage or current) arepredetermined in any conventional manner (for example, through normalcalibration or by accepting the manufacturer's calibration data). Thesignal conditioning necessary to implement the relationships is thenimplemented in the adapter 160. The conditioned signal is then appliedto the monitor 170 for processing (if needed) and display.

[0095] In some cases, the only signal conditioning required is scaling.This can be done using a conventional resistive network, with the sensoroutput signal forming the input and the system output signal being takenfrom an appropriate point in the network. Conventional passivecomponents may then be used to provide any necessary furtherconditioning such as impedance matching. This has the advantage ofimplementing the adapter 160 as a totally passive device. In othercases, conventional active components such as operational amplifierswith known resistive, capacitive and inductive feedback and feed-forwardelements may be used to implement the signal conversion.

[0096] In many cases, the relationship between sensor output andtemperature may be too irregular to implement accurately using purelypassive or analog components. In these cases, the adapter may beimplemented by including in the adapter 160 a conventionalanalog-to-digital converter (ADC), a microprocessor, and a memory; notethat a single conventional digital signal processor combines all thesefeatures in one component and may therefore in many applications be asuitable implementation. The relationship between the sensor output andtemperature can then be implemented as a look-up table in memory, or asparameters of an approximating function. Using known methods, themicroprocessor may then take as an input to the lookup table orapproximating function the sensed and ADC-converted sensor output signaland generate the corresponding temperature signal, which, after anyfurther conventional conditioning, is applied to the monitor 170.

[0097] In one embodiment of the invention that is particularly useful ina busy setting where only a quick and easy look at a patient'stemperature is needed, the entire conditioning, processing and displaycircuitry 150 is included in a single hand-held unit. In this case, thepower supply will typically be batteries and the monitor may be assimple as a conventional, low-power LCD display (along with conventionaldriving circuitry) showing temperature to, say, single decimalprecision.

[0098] Using such a self-contained, handheld device, a nurse wouldconnect the device to the temperature sensor by attaching the cable 190to the connector 180, and the patient's temperature would then bedisplayed on the display 174 in a predetermined format. The connector180 is preferably a conventional device such as a male/female plug pairthat would allow the nurse to quickly connect and disconnect the devicefor readings from different patients. This would allow the nurse to takereadings of many patients' temperatures quickly, with no need to waitfor a conventional thermometer to stabilize, and with little discomfortto the patients themselves. Indeed, the nurse could take an alreadycatheterized patient's temperature while he is asleep.

[0099] Assuming sufficiently powerful batteries, the self-containedembodiment of the system 150 could also include not only a memory, butalso a simple input device such as a button connected to an internalelectrical switch. Whenever the nurse presses the button, theinstantaneous measured temperature is stored in the memory portiondesignated for a predetermined number of values for the patient. A timestamp of the measurement could also be generated using known techniquesand stored along with each stored temperature measurement. By laterrecalling the stored values, for example by pressing the buttonaccording to some predetermined pattern, the nurse could then view thepatient's recent temperature history. The software and hardwarecomponents needed to implement this one-button storage and recallsystem, even classified for several different patients, may be similarto those used, for example, in conventional electronic hand bearingcompasses found on many well-equipped sailboats.

[0100] As an additional feature, the hand-held system could be providedwith conventional circuitry enabling it to download its storedtemperature information to another system such as a supervisory computeror patient monitor. The way in which such a feature is implemented isknown. The way in which such temperature values, time-stamped or not,are stored for one or more patients and then recalled for viewing on adisplay is also well known.

[0101] Several different embodiments of the invention have beendescribed above. It should be understood, however, that these are merelyillustrative. The invention is not to be limited to the particular formsor methods disclosed; rather, the invention is to cover allmodifications, equivalents and alternatives falling within the scope ofthe following claims.

What is claimed is:
 1. A vascular access device for measuringtemperature of a temperature medium of a patient, comprising: anelongated access device body sized to fit within a body vessel andincluding a proximal and a distal end, an outer wall, and at least onelumen within the outer wall comprising a thermal mass that can flow pasta predetermined location on the device body between the proximal anddistal ends; a temperature sensor supported by the device body at thepredetermined location and adjacent the outer wall thereof; and aninsulating gap formed in the access device adjacent to and inward fromthe temperature sensor and shaped to surround the temperature sensor onan inner side thereof so as to isolate the temperature sensor from theremainder of the device body including the lumen containing the thermalmass.
 2. The device as in claim 1, wherein the device body includes anopening in the outer wall at the predetermined location, and thetemperature sensor is located in the opening such that the outer wall isnot interposed between the temperature sensor and the fluid within thebody vessel.
 3. The device as in claim 2, further including pottingcompound securing the temperature sensor within the opening in the outerwall.
 4. The device as in claim 3, wherein the potting compound isdissolvable permitting the temperature sensor to directly contact thefluid within the body vessel.
 5. The device as in claim 1, wherein thedevice body includes an opening in the outer wall at the predeterminedlocation, and the temperature sensor is located externally to the outerwall of the device body adjacent the opening, the temperature sensorhaving a signal wire attached thereto that extends through the openingand through the device body to its proximal end.
 6. The device as inclaim 1, wherein the temperature sensor is mounted in a carrier thatextends along the device body within the outer wall thereof from theproximal end at least to the predetermined location, and wherein theinsulating gap is formed in the carrier.
 7. The device as in claim 1,wherein the insulating gap comprises a generally elongated slitextending along a portion of the length of the device body.
 8. Thedevice as in claim 1, wherein the insulating gap is expandable.
 9. Thedevice as in claim 1, wherein the device body is extruded and theinsulating gap is formed during the extrusion process as an insulatinglumen in the device body.
 10. The device as in claim 9, wherein theinsulating lumen is filled with a gas.
 11. The device as in claim 9,wherein the insulating lumen is filled with a substance selected fromthe group consisting of: ceramic pellets; a gel; and a piece ofinsulating material.
 12. The device as in claim 9, wherein theinsulating lumen extends along the device body at least from theproximal end to the predetermined location, and a plug is placed thereinproximal to the predetermined location.
 13. The device as in claim 1,wherein a plurality of insulating gaps are provided between thetemperature sensor and the lumen containing the thermal mass.
 14. Avascular access device for measuring temperature of a temperature mediumof a patient, comprising: an elongated access device body sized to fitwithin a body vessel and including a proximal and a distal end, an outerwall, and at least one lumen within the outer wall comprising a thermalmass past a predetermined location on the device body between theproximal and distal ends, and an opening in the outerwall at thepredetermined location; a flexible elongated member provided in thedevice body that extends across the opening in the outer wall; and atemperature sensor mounted within the flexible elongated member; andwherein the flexible elongated member is movable within the device bodysuch that the portion in which the temperature sensor is mounted may bedisplaced out of the opening thus creating an insulating gap between thetemperature sensor and the lumen containing the thermal mass.
 15. Thedevice as in claim 14, wherein a mid-portion of the flexible elongatedmember in which the temperature sensor is mounted is adapted to loop outof the opening.
 16. The device as in claim 14, wherein the temperaturesensor is mounted in the tip of the flexible elongated member which isbiased to curved outward through the opening.
 17. A device for measuringtemperature of a temperature medium of a patient, comprising: anextruded, elongated access device body sized to fit within a body vesseland including a proximal and a distal end, an outer wall, and a thermalmass at a predetermined location on the device body between the proximaland distal ends; a temperature sensor located at the predeterminedlocation on the device body; and an insulating structure of a differentmaterial than the device body and co-extruded with the device body thatextends between the temperature sensor and the thermal mass.
 18. Thedevice as in claim 17, wherein the insulating structure surrounds thetemperature sensor.
 19. The device as in claim 18, wherein theinsulating structure defines a tube that surrounds the temperaturesensor along at least a portion of the device body.
 20. The device as inclaim 17, wherein the thermal mass comprises a fluid within a lumendefined in the device body, and wherein the insulating structuresurrounds the lumen.